Impedance matching structure for slow wave device of microwave tube



Sept. 20, 1960 w. w. MENKE IMPEDANCE MATCHING STRUCTURE FOR SLOW WAVE DEVICE OF MICROWAVE TUBE Filed Nov. 17, 1958 O LO INVENTOR WARREN W NIENKE BY ATTORNEY Unitedf States Patent() llVIPEDAN CE MATCHING STRUCTURE FOR SLOW WAVE DEVICE OF MICROWAVE TUBE Filed Nov. 17, 195s, ser. N0.'774,z9s

10 claims. (C1. sis-,3.5)

This invention relates to microwave tubes utilizing a slow wave device for guiding electromagnetic energy for interaction with an electron beam, and it is particularly concerned lwith apparatus for supporting the slow wave device and matching it to a'microwave transmission line.

A slow wave device such as the helix of al travelling wave tube is generally matched to a coaxial line having a lower impedance than the helix." Sometimes. this is done by a horn having a throat portion and a flared mouth portion. The horn surrounds an end section of the helix and is connected to the outer conductor of the coaxial line. The inner conductorrof the coaxial line Vis connected to anend of the helix within the throat portionof the horn. The throat portion and the part of the helix therewithin form a .wire-above-ground-plane transmission line whose impedanceequals that of Vthe coaxial line. The flared mouth portion of the horn matches this impedance to the. higher impedance of the helix beyond the horn. .l

A fty ohm coaxial line is generally connected to the helixof a travelling wave tube. If the wire forming the helix has a diameter of the order of .020.inch, for example, and the dielectric constant vof the medium between the helixand horn is substantially equal to l, the spacing between thehelix and the throat portion of the horn must be ofthe order of .004 inch to provide a transmission line impedance of the order of ohms. It has heretofore been extremely diicult to attain and -accuratelymaintain such -a close spacing. Even if the required spacing were provided, short circuits might occur between the horn and helix, particularly if the travelling wave tube is subjected to mechanical shock and vibration.

Therefore, it is an object of the invention to provide a novel structure for supporting a slow Wave device of a microwave tube, and for matching this device to an external transmission line.

It is another object of the invention to provide a helix ,totransmission line matching structure wherein the helix is more readily supported in accurate spaced relationship with a horn of the matching structure than in previously known devices.

' .It is a further object to provide such a structure wherein the helix is ruggedly supported within the horn.

It is another object to provide a practical structure utilizing a horn for matching the impedance of a helix whose wire has a diameter of the order of a few hundredths of an inch, to the impedance of a coaxial line whose impedance is of the order of fifty ohms.

J It is yet a further object to provide a structure as aforedescribed that minimizes the possibility of the occurrence of short'circuits between the helix and horn.

Y Generally the foregoing objects are attained by suplporting an end section of the helix within a horn having athroat portion joined to a flared mouth portion by an 'inward step. VA sleeve of insulating material having a high dielectric constantis litted around the helix within 'the throat portion for maintaining an accurate spacing ICC . l 2 between the helix and the throat portion. A plurality of dielectric rods extend along the outside of the helix for supporting the helix within the mouth portion. O ne end of the sleeve and a group of ends ofthe dielectric rods lie adjacent each other at the'stepv lbetween the throat and mouth portions of the horn, this step enhancing the impedancematch provided by the structure. TheV details of the invention will become more apparent from the following description and accompanying drawings Wherein: v Y l j Fig. l is a sectionalview showing a travelling wave tube having input andoutput matching sections vin accord- -ance with the presentv invention; Y n Fig. 2 is across sectional view of the ktube shown in Fig. l, taken along the line 2%2; and

`Fig. 3 is a diagram for explaining how part of the matching section is designed electrically.

Referring now to `Fig. l,V a travelling wave tube is shoum having a cathode-focussing electrodesubassembly 11, and anode 13 and a collector 15. The anode and the Vcathode-focussing electrode subassembly are coaxially supported along the longitudinal axis of the tube byA a metallic cup-like element 17 forming part of a vacuum envelope for the tube; The collector 15` is coaxiallysupported along this axis by a'metallic cylinder 19 forming a further part of the vacuum envelope.

An apertured plate 21 extends across one, end ofthe cup-like element 17 for supporting thercylinder 19 in coaxial relationship with the anode 13. Plater 21 forms part of the vacuum envelope of the tube, and may be'lof highly magnetically permeable material for providing an input pole piece of a magnet arrangementy for ensuring that the diameter of the electronstream is maintained constantthroughout the slowwave propagating strucf ture of the tube. A plate 23 at the collector end of the tube constitutes an output pole piece'for the magnet'arrangement, not shown. v n

A wire helix ,25 isV coaxially supported along the axis of the tube between the anode 13 and the collector 15,. A conductive horn 27 encircles one end section of the helix at the input endl of the tube.V Thevh'orn 27 4passes through an aperture in plate 21 for support in coaxial relationship with the axis of Ythe tube. A conductive horn 29encircles the other end of the helix at the output end of the tube. Horn 29 -is provided with a ange 31 for coaxial support within cylinder 19. if

- AnY inputcoaxial line 33A passes into the tube through an aperture through vthe cylindrical wall of element 17; the coaxialline 33 being supported by element 17 'at right angles with'rthef axis of the tube. Therouter con? ductor of the coaxial'line is connected to the end of horn27 as illustratedin Fig. 1. A short linear extension 35 of the helix wire connects the inner conductor of coaxial line u33 directly tothe end turn of the helix.

An output coaxial lineV 37 lpasses' into the tube through an aperture provided in the cylindrical wall of the tube envelope 19. VCoaxial line 37 is also supported at right angles lwith the tube axis and is connected to the output ends of the helix 25 and horn 29 as illustrated.v The coaxial line 37 might instead pass out of the tubev along an axis parallel to the tube axis, provided a suitable right angle coaxial line bend isr employed. g

The input horn 27 has al throat portion 39 of constant inner diameter and a flared mouth portion 41 of tapered inner diameter. The throat and mouth portions of the horn are joined by an inward step 43 at right angles with the axis of the tube.A l

A dielectric sleeve45 is fitted around the end section of the helix 25 within the mouth portion 39 of the horn; The sleeve 45 maintains this section of the helix in rigid, coaxiallyspaced relationship Vwith the inner surface of the throat portion 39. The sleeve 45 preferablyhas a high dielectric constant. One end of the inner surface of sleeve 45 is tapered for providing a smooth transition to the flared mouth portion 41 of the horn 27. The output horn 29 is substantially the same as the horn 27; so will not be described in detail.

The intermediate region of the helix 25 is supported by a plurality of dielectric rods 49, 50 and 51 extending longitudinally along the outside of the helix between horns 27 and 29. These rods may be of the same delectric material as that of sleeve 43. The'diameters of rods 49-51 are equal to each other.V

The ends of the rods 49-51 are positioned within suitable slots in the mouth portions of the horns 27 and 29 so as to be disposed in 120 degree relationship with each other around the helix. The slots in the horn 27 are indicated by the numerals 54, 55 and 56 in Fig. 2.

VThe helix 25 and the rods 49-51 are confined within a longitudinally split cylinder having two halves 59 and 61 of conductive material. This cylinder is provided for clamping the rods in firm engagement with the helix, and for coaxially positioning the helix and rods subassembly along the axis of the tube. The ends of the cylinder are supported by the horns 27 and 29, respectively. A pair of semicircular members 63 and 64 clamp one end of the cylinder to the horn 27. A further pair of semicircular members 66 and 67 clamp the other end of the split cylinder to the horn 29.

One or more intermediate pairs of clamps suchas 71 and 73 are also provided yabout the split cylinder for further ensuring that the dielectric rods are pressed rmly against the helix. If intermediate regions along the rods 4951 are coated with attenuating material, as is often the case for a travelling wave amplifier, clamps 71 and 73 ensure that the split cylinder presses these intermediate attenuating regions into close contact with the `helix 25.

The aforedescribed tube is operated as a conventional travelling wave amplifier. The electron stream, which is produced by the cathode-focussing electrode subassembly 11 and the anode 13, is directed through the helix 25 for interaction with microwave energy travelling along the helix. The microwave energy supplied to the helix from coaxial line 33 is amplified by interaction with the electron stream. After amplification, it is transferred to the output coaxial line 37. The horn matching structures 27 and 29 ensure that there is a substantially refiectionless transfer of microwave energy to and from the helix over a wide frequency band.

It should be apparent that the ends of the helix are supported ruggedly within the horn matching structures. Therefore, mechanical shock or vibration of the tube is unlikely to produce a short circuit between the helix and either one of the horns. This is the case even if the helix has a small wire diameter, which requires that the spaces between the helix and the inner surfaces of the horns be small.

In order to more fully understand the construction and operation of the horn matching sections, the input matching section will now be described in further detail. First, the turns of helix 25 and the inner surface of the throat portion 39 of the horn 27 constitute a Wire-aboveground-plane transmission line. Its impedance should equal the characteristic impedance of coaxial line 33. Generally, this impedance is 50 ohms for a standard c0- axial line.

The impedance of a wire-above-ground-plane transmission line is given by the equation where h is the spacing between the wire and a ground plane conductor, e is the dielectric constant of a medium in which it isV assumed that the Wire is embedded, and

d is the diameter of the wire. Fig. 3 is a schematic diagram for illustrating how `the equation -is applied.

. If the `above equation is applied to the wire-above- 4 ground-plane transmission line of the input matching structure in Fig. 1, for example, the impedance of this transmission line can only be approximated, using the dielectric constant of sleeve 45 as e. This is the case since the helix wire is not actually embedded in the sleeve 45. For a given impedance, therefore, the thickness of sleeve 45 and the spacing between the helix 25 and the throat portion 39 of the horn is required to be slightly smaller than determined by the above equation.

The exact spacing for attaining la wire-above-groundplane transmission line impedance equal to that of input coaxial line 33 is determined empirically, after obtaining the approximate spacing from the above equation. In one design of a tube Where the diameter of the wire of helix 25 is .020 inch, the sleeve 45 having a dielectric constant of approximately 9, the thickness of sleeve 45 is .030 inch. This provides a wire-above-ground-plane impedance that is substantially 50 ohms.

The first few turns of the helix are stretched apart as indicated in Fig. 1 for enhancing the transfer of microwave energy from the straight wire extension 35 to the helix 25. The microwave energy travelling along the helix 25 within the horn throat portion 39 is primarily in a wire-above-ground-plane transmission line mode. The electric eld vectors of this mode extend between the helix wire and the inner surface of the throat portion 39 of the horn. Since the wire-above-ground-plane transmission line impedance substantially equals the impedance of coaxial line 33, microwave energy is eiiiciently transferred from the coaxial line 33 to the helix 25.

Generally, it would be desirable if the wire-aboveground-plane transmission line impedance at the input end of the horn mouth portion 41 be approximately equal to the transmission line impedance along the horn throat portion. However, the dielectric medium between the helix and horn within the horn mouth portion is constituted principally by a vacuum, whereas the medium between the helix and the throat portion of the horn is provided by sleeve 45. Sleeve 45 preferably has a much higher dielectric constant than the vacuum. There fore, the impedance of the helix at the input end of the horn mouth portion is higher than the impedance of the helix within the horn throat portion.

The step 43 is provided for reducing the impedance discontinuity effected by the change from one dielectric medium to another. Step 43 brings the inner surface of the smaller end of the horn mouth portion 41 into closer physical relationship with the helix 25 `than the inner surface of the horn throat portion 39. The magnitude of the step is limited by the minimum spacing that might be required to be maintained between the helix 25 and the inner surface of the horn at this point. In tubes where the diameter of the wire of helix 25 is of the order of .020 inch, it is impractical to extend the step 43 close enough to the helix 25 for attaining a transmission line impedance of 50 ohms. Therefore, the impedance at the input end of the horn mouth portion 41 will be slightly higher for a tube having such a small wire diameter.

The end of the sleeve 45 is tapered at 47 `for providing a smooth impedance transition from the 50 ohm wireabove-groundplane transmission line impedance to the slightly higher impedance at the input end of the horn mouth portion 41. The optimum length and angle of the taper 47 are best determined empirically.

The mouth portion 41 of the horn is flared for gradually changing the mode of energy travelling along the wire-above-ground-plane transmission line to a helix mode wherein the electric vectors of the energy extend primarily between the adjacent turns of the helix. At the same time, the impedance of the helix 25 is gradually increased along the horn mouth portion to a much higher Value at the output end ofthe horn. Although the taper of the horn is illustrated as being linear, it could instead be along an exponential curve.

The axial length of the mouth .portion 41 of the horn is not critical, except that it should be above some minimum length for energy at the lowest frequency of operation for the tube. This minimum length is best determined empirically since it is diliicult to determine the wavelength of energy within the horn mouth portion 41.

While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than of the limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. A travelling wave tube comprising a helix, means for producing and directing a stream of electrons through said helix, a horn enveloping one end of said helix, said horn having a throat portion of constant diameter and a ared mouth portion, a dielectric sleeve iitting around the helix within the throat portion of said horn, the outer surface of said sleeve being in contact with the inner surface of said throat portion along the length of said throat portion, one end of said sleeve being at the junction between the throat and the mouth portions of the horn, and a step along the inner surface of said horn at said junction for making the impedances of said helix on opposite sides of said junction approximately equal.

2. The combination as set forth in claim l, wherein the thickness of said dielectric sleeve is tapered in the vicinity of said step.

3. The combination as set forth in claim 2, further including a plurality of dielectric rods disposed around said helix beyond the mouth portion of said horn, said rods extending into said mouth portion for support of said helix therewithin, the ends of said rods being at the step between the throat and mouth portions of said horn.

4. In combination, a slow wave device for microwave energy having one of its ends surrounded by a tubular horn, said horn having a throat portion of constant diameter and a flared mouth portion, a dielectric sleeve fitting around said slow wave device within the throat portion of said horn for supporting said device in coaxially spaced relationship with said throat portion, the outer surface of said sleeve being in contact with the inner surface of said throat portion along the length of said throat portion and a plurality of dielectric rods extending from one end of said sleeve along the outside of said slow wave device for further supporting said device in coaxially spaced relationship with the mouth portion of said horn.

5. The combination as set forth in claim 4, wherein said throat portion is joined to said mouth portion by an inward step toward the axis of said horn, one of the ends of said sleeve and the ends of said dielectric rods adjacent said sleeve being located in the vicinity of said step.

6. The combination as set forth in claim 5, wherein the thickness of said dielectric sleeve is tapered at said one end thereof for providing an impedance transition l from the throat portion to the mouth portions of said horn.

7. The combination as set forth in claim 6, wherein the inner surface of said dielectric sleeve is tapered at said one end for substantially matching the taper of the inner surface of the flared mouth portion of said horn.

8. In a travelling wave tube, the combination of a helix whose ends are encircled by first and second conductive horns, respectively, each horn having a throat portion of constant inner diameter and a flared mouth portion, the inner surface of said throat portion being joined to the inner surface of the smaller end of said mouth portion by a step toward said helix, a plurality of dielectric rods extending longitudinally along the outside of said helix for supporting the helix in coaxially spaced relationship with the mouth portions of said horns, respectively, opposite ends of said rods being at the steps between the throat and flared mouth portions of said horns, respectively, means for rigidly supporting said rods relative to said horns, and rst and second dielectric sleeves fitting around opposite ends of said helix within the throat portions of said first and second horns, respectively, for supporting the ends of said helix in coaxially spaced relationship with the throat portions of said horns, the outer surfaces of said sleeves being in contact with the inner surfaces of the throat portions of said horns along the lengths of said throat portions, one of the ends of each sleeve being at the step between the throat and mouth portion of each horn.

9. The combination as set forth in claim 8, wherein the step between the throat and mouth portion of each horn is substantially at right angles with the longitudinal axis of each horn. Y

10. 'Ihe combination as set forth in claim 9, wherein the inner surface of each dielectric sleeve at the step between the throat and mouth portion of each horn is tapered for effecting a smooth impedance transition from the throat to the mouth portion of each horn.

References Cited in the iile of this patent UNITED STATES PATENTS 2,727,179 Lally et al. Dec. 13, 1955 2,794,143 Warnecke et al. May 28`,' 1957 2,862,137 Wang Nov. 25, 1958 2,887,608 McBee May 19, 1959 2,891,190 Cohn I-'une 16, 1959 

